The present invention relates to a branching unit for an optical transmission system. More particularly, the invention relates to apparatus for supplying such a system and branching unit with electrical power and a method of operation for that apparatus.
Optical transmission systems are utilised for sending data from one location to another. Typically, they utilise optical fibres for carrying a modulated light signal, the modulation being controlled according to the data being transmitted.
Such transmission systems can have high capacity and so are attractive for high volume communications systems. For this reason, amongst others, they are used for long distance communication systems, for example in submarine communications systems. Periodic amplification of the light signals is usually carried out using repeaters which require an electrical power feed.
In addition to repeaters, there are other devices in the optical transmission system which require electrical power. These include detectors for indicating the status of different parts of the system, such as different lengths of transmission line and branching units.
To supply power to the system, it is necessary to have an electrical conducting path along with the optical fibres. Usually, the conducting path is provided as a metallic sheath conductor around the bundle of optical fibres.
In order to provide a power supply of the desired magnitude, typically at a terminal of the transmission line, a power feed apparatus is used to hold the terminal at a particular voltage to feed power to the conductor. Typically, at the other terminal, a voltage of opposite polarity is applied.
In order to increase the reliability of a submarine optical transmission system, branching units are sometimes used. These allow a main, e.g. intercontinental, transmission cable to be connected to two landing transmission cables. In shallow waters and nearer land, damage to the transmission cables from trawlers, etc, is more likely than in deep water. Therefore two landing cables are used to increase the redundancy of the system. A failure or fault in one landing cable can be repaired whilst the data signals are routed along the other cable to and from the main cable.
Electrical current for powering electrical devices associated with the optical transmission system is conductable between a first branch of the branching unit and a main branch and between a second branch and the main branch.
In order to avoid overloading the conductor of the main cable, power is fed to the main cable from only one of the landing cables. However, in the event that a fault develops in that landing cable, it is necessary to de-power the system in order to repair the fault due to the high voltage nature of the power feed. This can result in a lengthy period of time in which the system is not operative, which is not desirable. In order to use the second landing cable to power the system, it is necessary to disconnect the first landing cable (for example at the branching unit) in order to avoid the power feed from the second landing cable flowing to e.g. an earth fault in the first landing cable. Such disconnection is clearly time-consuming and difficult if the branching unit is under water.